Slotted microstrip antennas are often used in aircraft and homing missiles. The antenna includes a bottom microstrip circuit including an array of microstrip lines of an electrically conductive metal, a dielectric layer in contact with and overlying said microstrip circuit, and metal ground plane in contact with and overlying the dielectric layer wherein the ground plane includes a metallic surface with rows of radiating slots etched through the metallic surface such that each row of slots overlies a particular microstrip line in said bottom microstrip circuit. These antennas offer high radiation efficiency and good side lobe control among other features. A difficulty with this type antenna however, is that the radiating slot elements are configured into the surface of the groundplane; the groundplane being a rather large metallic surface with the slots etched through the surface of the groundplane. This type of antenna presents a large, reflective metallic surface that can be easily located and targeted by hostile radar transceivers.
SUMMARY OF THE INVENTION
The general object of this invention is to provide an improved slotted microstrip antenna. A more particular object of the invention is to provide such an antenna that will exhibit a minimal radar target cross-section to hostile radar.
It has now been found that the aforementioned objects can be attained by applying a thin arc-plasma-sprayed, or flame sprayed coating of a ferrite material such as nickel zinc ferrite on the surface of the metal ground plane. The ferrite is not exposed to a high temperature anneal cycle after being flame sprayed thereby presenting a very high RF loss layer, absorbing nearly all RF radar signals impinging on it from hostile radars. Thus, any hostile radar will not receive a return of any signal it sent out and the ferrite coated antenna will be more or less invisible to the hostile radar.
The ferrite layer will be the last step in the fabrication of the antenna so that existing slotted microstrip antennas can employ this process. When applying the ferrite layer, the radiating slots will be masked by means of metal tape, silicone grease, etc. so that the slots will not be sprayed over. The masking will then be removed after the spray process.
One may also arc plasma spray nickel zinc ferrite material on to the ends of the microstrip lines to be terminated similarly as is accomplished in spraying this ferrite on the ground plane surface; both processes accomplishing the task of absorbing RF energy. The use of arc plasma spray in depositing the ferrite as load material is time-saving and economical in this case since the arc plasma spray process is already being used to coat the ground plane of the antenna and thus immediately lends itself to applying the load material to the ends of the microstrip lines without incurring any substantial effort. The circuit is merely flipped over and masked off so that the ferrite material can be sprayed onto the microstrip terminations.